Wound cell stack design for enhanced battery performance

ABSTRACT

An electrochemical cell comprising an electrode assembly in which overlayed electrodes are wound together in a bi-directional fashion yielding a high energy density cell stack with low internal impedance is described. The overlayed electrodes are such that either a single cathode is paired with two anodes or a single anode is paired with two cathodes prior to winding of the cell stack assembly. For example, the electrode assembly is formed by overlapping the two anode electrodes on opposite sides of the cathode electrode across a midportion and then winding the electrode strips bi-directionally about the midportion.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part application of Ser.No. 09/262,245, filed Mar. 4, 1999, now abandoned.

FIELD OF INVENTION

The present invention generally relates to the art of electrochemicalenergy, and more particularly, to an electrode assembly, electrochemicalcells in which the electrode assembly is used, and a method for makingthe electrode assembly.

BACKGROUND OF THE INVENTION

Batteries or electrochemical cells are typically volumetricallyconstrained systems that cannot exceed the available volume of thebattery case. The size and resulting volume of the battery case aredictated by the space requirements available for the particularapplication. The components that make up a battery, namely, the cathodeelectrode, the anode electrode, the separator, the current collectors,and the electrolyte all have to fit into the limited space defined bythe battery case. Therefore, the arrangement of the components impactson the amount of active electrode material that can be fit into the caseand the ease of manufacturing the unit.

Some typical electrode assemblies include the “Z” folded electrodeassembly that is disclosed in U.S. Pat. No. 3,663,721 to Blondel et al.In the “Z” folded electrode, a unitary and continuous lithium anode isfolded back and forth in a zigzag fashion. The length of the individualfolds determines the width of the electrode assembly. Individual cathodeplates are positioned between pairs of the pleated anode electrode andelectrically connected to one another. The design has some drawbacks,including the requirement that separate cathode plates be insertedbetween each pair of adjacent layers of anode electrode and therequirement that electrical connections be made between all of theinserted cathode plates. This arrangement increases the time and costsassociated with manufacturing.

Another typical electrode assembly configuration is the “jelly roll”design in which the anode electrode, the cathode electrode, and theseparator are overlaid with respect to each other and coiled up. Such anelectrode configuration is desirable because the continuous anode andcathode electrodes require a minimal number of mechanical connections totheir respective terminal leads, and the jelly roll assembly isgenerally recognized as preferred for high discharge and current pulseapplications. However, in some applications, a cylindrically shapedelectrode assembly is not desired because of other factors, such as theshape of the battery case.

U.S. Pat. No. 4,761,352 to Bakos et al. discloses yet another electrodeassembly design comprising an accordion folded electrode assembly withunitary members for both the anode and cathode strips. The cathode stripis approximately half the length of the anode strip, and the anode stripis folded over the cathode strip to “sandwich” the cathode between twolayers of the anode. The resulting form is then manually folded in analternating series of “V” folds (best shown in FIG. 4 of the patent).However, that design provides some undesirable gaps which reduce thevolumetric density of the electrochemically active materials.

What is needed is an improved multi-layer, folded electrode assemblydesign for high energy devices that includes many of the desirablefeatures of the jelly roll design, such as unitary anode and cathodeelectrodes.

SUMMARY OF THE INVENTION

The present invention fills the above-described need by providing anelectrochemical cell comprising an electrode assembly in which theelectrodes are wound together in a bi-directional fashion, yielding ahigh energy density cell with low internal impedance. The anode andcathode electrodes are arranged in the cell in such a fashion thatprovides efficient utilization of the active components. The resultantwound assembly is configured such that it can be conveniently packagedin either a cylindrical or prismatic housing.

In one embodiment of the electrochemical cell, the electrodes areprovided as two anode assemblies and one cathode assembly configuredsuch that each anode is positioned on either side of the cathodeassembly, and extending in opposing directions. At the center mostportion of the assembly there is an overlap of anodes. This assembly isthen wound about the overlapping region in a bi-directional fashion. Theresultant assembly produces a wound cell stack configuration with auniform contact of anode and cathode, such that the cell is balancedelectrochemically and provides for optimum volume utilization within thebattery enclosure. Each anode has one or more tabs that can be welded tothe case. Alternately, two cathode assemblies can be paired with oneanode assembly, with a resultant cathode tab welded to the case. In bothof the above configurations, the opposite electrode may contain one ormore tabs which are then electrically connected to the batteryfeedthrough pin.

An alternate embodiment of this invention provides for an anodeelectrode and a cathode electrode, wherein the electrodes are slotted.The electrodes are inserted, one into the other, essentially forming an“X”. Upon collapsing the electrodes, a variation of the above-describedinvention is obtained wherein the anode is approximately equallydisposed on opposite sides of the cathode, radiating outwardly from itsmidportion. This assembly is then wound from the center, resulting in apreferred cell stack assembly. This configuration provides for theadditional advantage of having the anode registered to the cathode, andmitigates the need for aligning two distinct anodes to the cathode.

Other features and advantages of the present invention will becomeapparent upon reading the following detailed description of embodimentsof the invention, when taken in conjunction with the accompanyingdrawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of the cathode strip and separator ofthe present invention;

FIG. 2 is a side elevational view of the anode strip and separator ofthe present invention;

FIG. 3 is a bottom plan view of the cell stack assembly of the presentinvention;

FIG. 4 is a side elevational view of the cell stack assembly of thepresent invention;

FIG. 5 is a partial plan view of the wound electrode assembly of thepresent invention;

FIG. 6 is a perspective view of an alternate embodiment of the electrodestrips of the present invention;

FIG. 7 is a partial plan view of the wound electrode assembly of thealternative embodiment; and

FIG. 8 is an exploded view of an electrochemical cell of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is designed for high energy devices such asbatteries and capacitors and is adaptable in a wide variety of electrodeconfigurations and shapes for applications as capacitors and batteries,including aqueous and nonaqueous primary and secondary batteries.

Referring to FIG. 1, a first electrode 10 is preferably a continuousstructure comprising an active material 11 contacted to a currentcollector 12 (shown in dashed lines). The active material for a cathodeelectrode is preferably comprised of a metal, a metal oxide, a metalsulfide, a mixed metal oxide, a carbonaceous material, or the like andis combined with the current collector of a conductive material such asa conductive screen. For an anode electrode, the preferred activematerial is an alkali metal selected from Group 1A of the Periodic Tableof Elements and contacted to an anode current collector. A preferredanode electrode comprises lithium contacted to a nickel currentcollector. In a preferred form of the present invention, the electrodestrip 10 is a cathode electrode having a set of cathode tabs 15 providedfor making an electrical connection to a positive terminal.

Turning to FIGS. 2 and 3, a second electrode 16 includes a pair ofsecond electrode strips of a second electrode active material 17contacted to a current collector 18 (shown in dashed lines) disposed onopposite sides of the first electrode 10. The second electrode strips 16overlap along a midportion 19 of the first electrode 10 (FIG. 3).Preferably, the second electrode strips 16 are part of the anodeelectrode. The anode electrode strips 16 have anode tabs 22 that providefor electrical connection to a negative terminal.

As shown in FIGS. 1, 2 and 4, a separator material 13 is disposed behindeach elect-rode to prevent contact between overlayed layers ofelectrodes. Alternatively, the separator 13 is disposed in front of eachelectrode strip. In a preferred embodiment, which is not shown in thedrawings, a separator 13 in the form of an envelope encapsulates each ofthe first and second electrodes 10, 16. In that respect, whether theseparator 13 is disposed between immediately adjacent electrode stripsor, the separator serves as an envelope encapsulating at least one ofthe electrodes, the separator must prevent direct physical contactbetween the electrodes 10, 16.

Turning to FIG. 4, an electrode assembly according to the presentinvention comprises a cathode electrode 10 and two anode electrodes 16A,16B, which are each preferably elongate, flat, and rectangular. Theanode electrodes 16A, 16B are disposed on opposite sides of the cathode10 and aligned such that they overlap across the midportion 19 thereof.The anode electrodes 16A, 16B are a little more than half the length ofthe cathode electrode 10, and extend a short distance across themidportion 19 in order to overlap. Alternately, two cathode electrodeassemblies are paired with one anode electrode in a similar overlappingconfiguration.

From the alignment shown in FIGS. 3 and 4, the electrode strips 10 and16 are then folded about the overlapping region in a bi-directionalfashion to provide the electrode assembly 25. As shown in FIG. 5, thoseportions of anode strips 16A and 16B on the outside of the assembly 25have the outside of the current collector devoid of anode activematerial. This is because there is no opposing cathode active material,and such anode active material would provide very little, if any,additional volumetric efficiency. Also, the ends of the anode strips 16Aand 16B extend somewhat beyond the end of the cathode electrode 10 tofully utilize the discharge efficiency of the cathode electrode.

The term bi-directional refers to the fact that one side is foldeddownwardly and the opposite side is folded upwardly, either insuccession or simultaneously, to generate the electrode assembly 25shown in FIG. 5. The electrode assembly 25 produces a wound cell stackconfiguration with uniform contact of anode and cathode electrodes suchthat the cell is balanced electrochemically and provides for optimumvolume utilization within the battery enclose.

An alternate embodiment of the present invention is shown in FIGS. 6 and7. In this embodiment, a cathode electrode strip 50 comprising a cathodeactive material 52 contacted to a cathode current collector 54 has adownwardly facing slot 53 disposed in a midportion 56 thereof. An anodeelectrode strip 60 comprises an anode active material 62 contacted to ananode current collector 64 and includes an upwardly facing slot 63disposed in a midportion 66. The strips 50 and 60 are moved togetherwith the slots 53, 63 registering with each other to form a collapsibleX-shaped assembly. In this embodiment, the anode strip 60 extendsoutwardly a small distance past the opposed ends of the cathode strip 50and in a configuration such that the electrodes 50, 60 radiate outwardlyfrom the midportions 56, 66 of the other electrode. The electrode strips50, 60 are then folded in a bi-directional fashion from the center ormidportions 56, 66 to produce the wound electrode assembly 75 shown inFIG. 7. The bi-directional folding is similar to that described withrespect to the electrode assembly 25 shown in FIGS. 1 to 5.

The completed electrode assembly 75 shown in FIG. 7 is similar to theelectrode assembly 25 in the respect that those portions of anode strip60 on the outside of the assembly have the outside of the currentcollector devoid of anode active material. As previously explained, thisis because there is no opposing cathode active material there, and suchanode active material would provide very little, if any, additionalvolumetric efficiency. Also, the ends of the anode strip 60 extendsomewhat beyond the respective ends of the cathode strip 50 to fullyutilize the discharge efficiency of the cathode electrode. Thisalternate embodiment provides the additional advantage of having theanode registered to the cathode and mitigates the need for aligning twodistinct anodes to the cathode.

The present electrode assemblies 25, 75 provide several advantages tocell design, including high energy density with low internal impedance.Additionally, the anode and cathode electrodes 10, 16 for assembly 25and the electrodes 50, 60 for assembly 75 are arranged in the cell in away that provides efficient utilization of the active components. Theresultant wound cell stacks are configured such that they can beconveniently packaged in either a cylindrical or prismatic shapedcasing. These casing shapes are well known to those of ordinary skill inthe art. The electrode assemblies 25, 75 also provide a cell stackconstruction in which the anode and cathode are uniformly utilizedduring cell discharge. Finally, the assemblies 25, 75 provide a cellhaving a relatively high inter electrode surface area which results in ahigh current rate capability. This is advantageous for use inapplications such as powering an implantable defibrillator.

A preferred primary electrode chemistry for the electrode assemblies 25,75 according to the present invention has the first electrode 10, 50 ofa mixed metal oxide such as silver vanadium oxide (SVO), copper silvervanadium oxide (CSVO) or a fluorinated carbonaceous material (CF_(x)),and the second electrode 16, 60 comprising lithium. A Li/SVO or Li/CSVOelectrochemical couple is activated with an electrolyte of 0.25 M to 1.5M LiAsF₆ or LiPF₆ in a 50:50, by volume, mixture of propylene carbonateand 1,2-dimethoxyethane. For a Li/CF_(x) cell, the preferred electrolyteis 1.0 M to 1.4 M LiBF₄ in γ-butyrolactone. A preferred secondarychemistry has a carbonaceous negative electrode and a lithiated counterelectrode. A preferred lithiated material is lithium cobalt oxide. Thiscouple is activated with an electrolyte of 1 M LiPF₆ or 1 M LiAsF₆ inethylene carbonate/1,2-dimethoxyethane (3:7).

Referring to FIGS. 1, 2 and 8, the anode tabs 22 can be welded to thecase 80 (negative). Alternately, two cathode assemblies can be pairedwith one anode assembly with the resultant cathode tabs (not shown)welded to the case 80 (positive). In both of the above configurations,the opposite electrode may contain one or more tabs (cathode tabs 15)that are electrically connected to the battery feedthrough or terminalpin 82. The terminal pin 82 is electrically insulated from the lid 84 ofthe casing 80 by a glass-to-metal seal 86. Similar electricalconnections for the cathode strip 50 and the anode strip 60 are made forthe electrode assembly 75 shown in FIGS. 6 and 7.

While the invention has been described in connection with certainpreferred embodiments, it is not intended to limit the scope of theinvention to the particular forms set forth, but, on the contrary, it isintended to cover such alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention, as definedby the appended claims.

What is claimed is:
 1. An electrode assembly, comprising: a) a firstelectrode strip having a midportion and a predetermined first length; b)at least two second electrode strips having a second length shorter thanthe first length, wherein the at least two second electrode strips aredisposed on opposite sides of the first electrode strip and in anoverlapping fashion along the midportion of the first electrode strip,and c) a separator material disposed between the first electrode stripand the at least two second electrode strips, wherein the firstelectrode strip and the at least two second electrode stripsbi-directionally foldable about the midportion of the first electrodestrip such that a wound cell stack is formed.
 2. The electrode assemblyof claim 1 wherein the at least two second electrode strips each have atleast one tab for connection to a battery case.
 3. The electrodeassembly of claim 1 wherein the first electrode strip has a tabconnectable to a terminal pin.
 4. The electrode assembly of claim 1wherein the first electrode strip comprises an anode electrode and theat least two second electrode strips comprise a cathode electrode. 5.The electrode assembly of claim 1 wherein the first electrode stripcomprises a cathode electrode and the at least two second electrodestrips comprise an anode electrode.
 6. The electrode assembly of claim 1wherein the first electrode strip is a unitary member.
 7. The electrodeassembly of claim 1 wherein the at least two second electrode strips areunitary members.
 8. The electrode assembly of claim 1 of either aprimary or a secondary chemistry.
 9. The electrode assembly of claim 1wherein the first electrode strip is of a first electrode activematerial selected from the group consisting of SVO, CSVO and CF_(x), andthe at least two second electrode strips are of a second electrodeactive material comprising lithium.
 10. The electrode assembly of claim1 wherein the first electrode strip is of a first electrode activematerial comprising lithium cobalt oxide, and the at least two secondelectrode strips are of a second electrode active material comprising acarbonaceous material.
 11. An electrochemical cell, comprising: (a) acathode strip comprising a cathode active material contacted to acathode current collector and having a midportion and a predeterminedfirst length; (b) at least two anode strips comprising lithium contactedto an anode current collector and having a second length shorter thanthe first length, wherein the at least two anode strips are disposed onopposite sides of the cathode strip in an overlapping fashion along themidportion of the cathode strip; (c) a separator material disposedbetween the cathode strip and the at least two anode strips; and (d) anelectrolyte activating and operatively associating the cathode strip andthe anode strips, wherein the cathode strip and the anode strips arebi-directionally foldable about the midportion of the cathode strip toform a wound cell stack.
 12. The electrochemical cell of claim 11wherein the at least two anode strips each have at least one tab forconnection to a battery case.
 13. The electrochemical cell of claim 11wherein the cathode strip has a tab connectable to a terminal pin. 14.The electrochemical cell of claim 11 of either a primary or a secondarychemistry.
 15. The electrochemical cell of claim 11 wherein the cathodeactive material of the cathode strip is selected from the groupconsisting of SVO, CSVO and CF_(x), and the anode active material of theat least two anode strips comprises lithium.
 16. The electrochemicalcell of claim 11 wherein the cathode active material of the cathodestrip comprises lithium cobalt oxide, and the anode active material ofthe at least two anode strips comprises a carbonaceous material.
 17. Theelectrochemical cell of claim 11 wherein the cathode strip is a unitarymember.
 18. The electrochemical cell of claim 11 wherein the at leasttwo anode strips are unitary members.